Things like emoji usage, page navigations, feature uses, etc. Ideally anonymous; no IP, user agent, etc, just a small byte or two packed properly can go a long way.
The problem is that it's actually quite hard to reliably anonymize data especially once you start to begin combining data sets from multiple places. That's the problem differential privacy is trying to solve in a mathematically rigorous way.
See for example how researchers partially de-anonymized Netflix Prize data by cross-referencing it with IMDB reviews.
DDG:
{ used_advanced_search }
{ used_country_toggle }
{ tabbed, *tab_maps }
{ filtered, *filter_date }
{ os "iOS", *ver "13.5", browser "Safari" }
Mail
{ disabled_remote_images }
{ flagged_mail }
Keyboard
{ emoji_keyboard_via_globe }
{ *emoji_use "100-1000", *emojis [ ":)" ":P" ":(" ] }